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    <title>Building legends in d3.js</title>
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    <meta name="author" content="Yan Holtz" />
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      <div class="textlanding">
        <h1>Building legends in d3.js</h1>
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        <br /><br />
        <p style="max-width: 700px; margin: auto">
          It is a common and necessary practice in data visualization to build
          legends. D3.js does not provide any helper function for that, meaning
          you have to build it from scratch. This document provides a few
          templates for categorical and continuous legend. Hopefully you will
          find one that suits your needs.
        </p>
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        <ul class="nav nav-pills flex-column">
          <li class="nav-item">
            <i class="nav-link" style="opacity: 0.5">Categorical</i>
          </li>
          <li class="nav-item">
            <a class="nav-link" href="#cat1">100% hand made</a>
          </li>
          <li class="nav-item">
            <a class="nav-link" href="#cat2">Use a loop</a>
          </li>
          <li class="nav-item">
            <a class="nav-link" href="#cat3">Square</a>
          </li>
          <li class="nav-item">
            <i class="nav-link" style="opacity: 0.5">Continuous</i>
          </li>
          <li class="nav-item">
            <a class="nav-link" href="#cont1">template 1</a>
          </li>
          <li class="nav-item">
            <a class="nav-link" href="#cont2">template 2</a>
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            <a class="nav-link" href="#cont3">template 3</a>
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    <!-- ==================== CAT 1 ==================== -->

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      <div class="container">
        <h1>Categorical legend: 100% handmade</h1>
        <hr />
        <div class="row">
          <!-- ==================== GRAPH SECTION = WHERE THE GRAPH APPEARS ==================== -->
          <div class="col-lg-6 .align-middle">
            <div style="position: -webkit-sticky; position: sticky; top: 120px">
              <div class="bg-light" id="res-sectionA"></div>
              <br />
              <div>
                <h5>Notes:</h5>
                <ul>
                  <br />
                  <li>
                    Each element of the legend is added with a specific line of
                    code.
                  </li>
                  <br />
                  <li>Very tedious, but allows fine grain customization.</li>
                  <br />
                  <li>
                    Not suitable if many groups since it would create too much
                    code.
                  </li>
                </ul>
              </div>
            </div>
          </div>

          <!-- ==================== CODE SECTION = WHERE THE CODE APPEARS ==================== -->
          <div class="col-lg-6 text-center .align-middle">
            <!-- ========= show html code ============== -->
            <aside
              style="position: -webkit-sticky; position: sticky; top: 120px"
            >
              &larr; Edit me!
            </aside>
            <pre
              class="language-html"
            ><code id="html-sectionA" contenteditable="true"><xmp><!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="https://d3js.org/d3.v4.js"></script>

<!-- Create a div where the graph will take place -->
<div>
  <svg id="my_dataviz" height=300 width=450></svg>
</div>
</xmp></code></pre>
            <!-- ==================================== -->

            <!-- ========= show JS code ==============  -->
            <pre
              class="language-js"
            ><code id="js-sectionA" contenteditable="true"><xmp><script>

// select the svg area
var svg = d3.select("#my_dataviz")

// Handmade legend
svg.append("circle").attr("cx",200).attr("cy",130).attr("r", 6).style("fill", "#69b3a2")
svg.append("circle").attr("cx",200).attr("cy",160).attr("r", 6).style("fill", "#404080")
svg.append("text").attr("x", 220).attr("y", 130).text("variable A").style("font-size", "15px").attr("alignment-baseline","middle")
svg.append("text").attr("x", 220).attr("y", 160).text("variable B").style("font-size", "15px").attr("alignment-baseline","middle")

</script></xmp></code></pre>
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            <script>
              // At the beginning, I read the html and JS fragments
              myHtmlParser("html-sectionA", "res-sectionA");
              myJSParser("js-sectionA", "res-sectionA");

              // If the user change the JS fragment, I run it again:
              document
                .getElementById("js-sectionA")
                .addEventListener("input", function () {
                  d3.select("#res-sectionA").html("");
                  myHtmlParser("html-sectionA", "res-sectionA");
                  myJSParser("js-sectionA", "res-sectionA");
                });

              // If the user change the HTML fragment, I run it again:
              document
                .getElementById("html-sectionA")
                .addEventListener("input", function () {
                  d3.select("#res-sectionA").html("");
                  myHtmlParser("html-sectionA", "res-sectionA");
                  myJSParser("js-sectionA", "res-sectionA");
                });
            </script>
          </div>
        </div>
      </div>
    </section>
    <br />

    <!-- ==================== USE A LOOP ==================== -->

    <section id="cat2" class="bg">
      <div class="container">
        <h1>Categorical legend: use a loop</h1>
        <hr />
        <div class="row">
          <!-- ==================== GRAPH SECTION = WHERE THE GRAPH APPEARS ==================== -->
          <div class="col-lg-6 .align-middle">
            <div style="position: -webkit-sticky; position: sticky; top: 120px">
              <div class="bg-light" id="res-sectionB"></div>
              <br />
              <div>
                <h5>Notes:</h5>
                <ul>
                  <br />
                  <li>
                    If you have legends with many entry, it is more convenient
                    to use a loop.
                  </li>
                  <br />
                  <li>
                    This is done using a classic <code>enter()</code> approach:
                    adding one element per entry of the key.
                  </li>
                </ul>
              </div>
            </div>
          </div>

          <!-- ==================== CODE SECTION = WHERE THE CODE APPEARS ==================== -->
          <div class="col-lg-6 text-center .align-middle">
            <!-- ========= show html code ============== -->
            <aside
              style="position: -webkit-sticky; position: sticky; top: 120px"
            >
              &larr; Edit me!
            </aside>
            <pre
              class="language-html"
            ><code id="html-sectionB" contenteditable="true"><xmp><!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js & color palette -->
<script src="https://d3js.org/d3.v4.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>

<!-- Create a div where the graph will take place -->
<div>
  <svg id="my_dataviz2" height=300 width=450></svg>
</div>
</xmp></code></pre>
            <!-- ==================================== -->

            <!-- ========= show JS code ==============  -->
            <pre
              class="language-js"
            ><code id="js-sectionB" contenteditable="true"><xmp><script>

// select the svg area
var Svg = d3.select("#my_dataviz2")

// create a list of keys
var keys = ["Mister A", "Brigitte", "Eleonore", "Another friend", "Batman"]

// Usually you have a color scale in your chart already
var color = d3.scaleOrdinal()
  .domain(keys)
  .range(d3.schemeSet2);

// Add one dot in the legend for each name.
Svg.selectAll("mydots")
  .data(keys)
  .enter()
  .append("circle")
    .attr("cx", 100)
    .attr("cy", function(d,i){ return 100 + i*25}) // 100 is where the first dot appears. 25 is the distance between dots
    .attr("r", 7)
    .style("fill", function(d){ return color(d)})

// Add one dot in the legend for each name.
Svg.selectAll("mylabels")
  .data(keys)
  .enter()
  .append("text")
    .attr("x", 120)
    .attr("y", function(d,i){ return 100 + i*25}) // 100 is where the first dot appears. 25 is the distance between dots
    .style("fill", function(d){ return color(d)})
    .text(function(d){ return d})
    .attr("text-anchor", "left")
    .style("alignment-baseline", "middle")
</script></xmp></code></pre>
            <!-- ==================================== -->

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            <script>
              // At the beginning, I read the html and JS fragments
              myHtmlParser("html-sectionB", "res-sectionB");
              myJSParser("js-sectionB", "res-sectionB");

              // If the user change the JS fragment, I run it again:
              document
                .getElementById("js-sectionB")
                .addEventListener("input", function () {
                  d3.select("#res-sectionB").html("");
                  myHtmlParser("html-sectionB", "res-sectionB");
                  myJSParser("js-sectionB", "res-sectionB");
                });

              // If the user change the HTML fragment, I run it again:
              document
                .getElementById("html-sectionB")
                .addEventListener("input", function () {
                  d3.select("#res-sectionB").html("");
                  myHtmlParser("html-sectionB", "res-sectionB");
                  myJSParser("js-sectionB", "res-sectionB");
                });
            </script>
          </div>
        </div>
      </div>
    </section>
    <br />

    <!-- ==================== SQUARE ==================== -->

    <section id="cat3" class="bg">
      <div class="container">
        <h1>Categorical legend: square</h1>
        <hr />
        <div class="row">
          <!-- ==================== GRAPH SECTION = WHERE THE GRAPH APPEARS ==================== -->
          <div class="col-lg-6 .align-middle">
            <div style="position: -webkit-sticky; position: sticky; top: 120px">
              <div class="bg-light" id="res-sectionC"></div>
              <br />
              <div>
                <h5>Notes:</h5>
                <ul>
                  <br />
                  <li>Small modification to add squares instead of circles.</li>
                  <br />
                  <li>
                    Play with the <code>size</code> variable to change the whole
                    size.
                  </li>
                </ul>
              </div>
            </div>
          </div>

          <!-- ==================== CODE SECTION = WHERE THE CODE APPEARS ==================== -->
          <div class="col-lg-6 text-center .align-middle">
            <!-- ========= show html code ============== -->
            <aside
              style="position: -webkit-sticky; position: sticky; top: 120px"
            >
              &larr; Edit me!
            </aside>
            <pre
              class="language-html"
            ><code id="html-sectionC" contenteditable="true"><xmp><!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js & color palette -->
<script src="https://d3js.org/d3.v4.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>

<!-- Create a div where the graph will take place -->
<div>
  <svg id="my_dataviz3" height=300 width=450></svg>
</div>
</xmp></code></pre>
            <!-- ==================================== -->

            <!-- ========= show JS code ==============  -->
            <pre
              class="language-js"
            ><code id="js-sectionC" contenteditable="true"><xmp><script>

// select the svg area
var SVG = d3.select("#my_dataviz3")

// create a list of keys
var keys = ["Mister A", "Brigitte", "Eleonore", "Another friend", "Batman"]

// Usually you have a color scale in your chart already
var color = d3.scaleOrdinal()
  .domain(keys)
  .range(d3.schemeSet1);

// Add one dot in the legend for each name.
var size = 20
SVG.selectAll("mydots")
  .data(keys)
  .enter()
  .append("rect")
    .attr("x", 100)
    .attr("y", function(d,i){ return 100 + i*(size+5)}) // 100 is where the first dot appears. 25 is the distance between dots
    .attr("width", size)
    .attr("height", size)
    .style("fill", function(d){ return color(d)})

// Add one dot in the legend for each name.
SVG.selectAll("mylabels")
  .data(keys)
  .enter()
  .append("text")
    .attr("x", 100 + size*1.2)
    .attr("y", function(d,i){ return 100 + i*(size+5) + (size/2)}) // 100 is where the first dot appears. 25 is the distance between dots
    .style("fill", function(d){ return color(d)})
    .text(function(d){ return d})
    .attr("text-anchor", "left")
    .style("alignment-baseline", "middle")
</script></xmp></code></pre>
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              // If the user change the JS fragment, I run it again:
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                .addEventListener("input", function () {
                  d3.select("#res-sectionC").html("");
                  myHtmlParser("html-sectionC", "res-sectionC");
                  myJSParser("js-sectionC", "res-sectionC");
                });

              // If the user change the HTML fragment, I run it again:
              document
                .getElementById("html-sectionC")
                .addEventListener("input", function () {
                  d3.select("#res-sectionC").html("");
                  myHtmlParser("html-sectionC", "res-sectionC");
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